Samantha’s Runtime Ratio Rule The Only Reliable Way to Tell If Your Current System Is Oversized

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If you’ve ever wondered whether your AC or heat pump is too big for your home, you’re not alone. Oversizing is the most common HVAC problem in modern homes, especially in newer builds, open-concept layouts, and homes that have undergone insulation upgrades or window replacements.

But here’s what most homeowners don’t realize:

👉 You can’t tell if your system is oversized by looking at the model number, the tonnage label, or your square footage.

The only reliable way to know is by measuring its Runtime Ratio (RR%) — a real-world diagnostic method I’ve used for years. It works on any system (old, new, R-410A, R-32, heat pump, central AC) and is shockingly accurate.

By the end of this guide, you’ll know:

Why oversizing is so common
What Runtime Ratio really measures
How to measure RR% with nothing but a smart sensor
What RR% tells you about comfort, humidity, and sizing
How to use this data before buying a new system

And most importantly…

You’ll know with real data whether your current AC is too big, too small, or just right.

1️⃣ 🏠 Why Oversizing Happens (Even in New Homes)

Most homeowners assume bigger means better — especially when they’ve lived in an uncomfortable house before. But oversizing is caused by four specific problems:

📦 Problem 1: Contractors Oversize for “Safety”

Many installers add an extra ton “just to be safe.”
But oversizing increases heating/cooling problems instead of reducing them.

📏 Problem 2: Sizing Charts Rely on Square Footage Only

Charts like “1 ton per 600 sq ft” are outdated.

Modern homes vary massively in:

insulation
windows
air leakage
orientation
ceiling height
solar exposure

🏠 Problem 3: Homes Become Tighter Over Time

Adding:

new windows
more attic insulation
air sealing
spray foam
R-38+ attic upgrades

…reduces the needed tonnage by ½–1 ton, but most homeowners simply reinstall the same size they previously had.

🔧 Problem 4: Old Systems Were Oversized to Begin With

If your home was built before 2005, there’s a good chance the original furnace or AC was already bigger than needed.

When you replace a 20-year-old 4-ton unit with a new 4-ton unit…

You’re repeating the original mistake.

**2️⃣ ⏱️ What Is Runtime Ratio (RR%)?

The Single Most Accurate Sizing Test**

Runtime Ratio tells you:

➡ How much of each hour your system actually runs on a hot day.

It’s simple math:

Runtime minutes ÷ total minutes in the hour × 100 = Runtime Ratio%

Example:
AC runs 24 minutes out of 60 → RR% = 40%.

And here’s why this matters:

👉 Oversized systems run too little.
👉 Undersized systems run too much.

Runtime Ratio cuts through all the guesswork.

External Verified Source:

EPA on HVAC cycle and humidity behavior

3️⃣ 📱 How to Measure Runtime Ratio With a Smart Sensor

You only need one basic tool:

A temp + humidity smart sensor
(or thermostat with runtime history)

Place the sensor:

In a main living space
Out of sunlight
Near thermostat height (5 ft)
Away from supply vents

Let it record:

Temperature patterns
Humidity patterns
Cycle times (on/off behavior)

Ideal Measurement Day

Choose a day that hits:

88–95°F outdoor temp (for cooling tests)
20°F below outdoor temp for heating tests

Humidity should be above 50%.

This gives you a true, full-load test.

4️⃣ 🔍 Samantha’s Runtime Ratio Thresholds (The Gold Standard)

Here is the truth HVAC pros don’t tell you —
these numbers never lie.

💙 If RR% is LESS than 30% → Your system is OVERSIZED

Symptoms include:

Humidity stays high
Rooms cycle from cold to warm
AC cools quickly but comfort feels “off”
Thermostat hits temp too fast
Short cycling
Hot rooms that never stabilize
Increased energy bills
Coil freeze in some cases

Oversized units remove less humidity and create rapid cycling.

💚 If RR% is 40–60% → Your system is PERFECTLY SIZED

This is the sweet spot.

You’ll see:

Stable temps
Good humidity control
Even cooling
Quiet operation
Consistent comfort
Lower energy bills

Systems in this range last the longest.

❤️ If RR% is 70–95% → Your system is UNDERSIZED

Symptoms include:

Long, endless cycles
Stays on during hottest hours
Slower temperature recovery
High indoor humidity
Upstairs stays warmer

This is when a tonnage increase or duct fix is justified.

5️⃣ 💧 What Runtime Ratio Reveals That Sizing Charts Can’t

Here’s what RR% can diagnose that no other test can:

✔ Humidity Removal Efficiency

Oversized systems run too short to remove moisture.
RR% below 30% = humidity problems guaranteed.

External Verified Source:

ENERGY STAR on humidity and AC operation

✔ Solar Gain Problems (Hot Rooms)

If RR% is normal overall but one room is hot,
the system is correctly sized —
but ductwork or sun exposure is the issue.

✔ Ductwork Restriction

Low Delta-T + long cycles = airflow problem
—not a tonnage issue.

✔ Oversizing Masked as Poor Airflow

If RR% is low but humidity is high,
you have an oversized and airflow-restricted system.

✔ True System Capacity Loss

RR% > 85% on hot days often means:

weak compressor
low refrigerant charge
evaporator coil airflow issues
or insufficient ductwork

RR% exposes these instantly.

6️⃣ 🔧 How to Perform Samantha’s 24-Hour RR% Audit

This is my signature process — used for every homeowner I help diagnose.

🌅 Step 1 — Morning Baseline (8 AM–12 PM)

Record RR% as the day warms.
Should slowly increase.

☀️ Step 2 — Peak Heat Test (12 PM–6 PM)

This is the true sizing zone.

RR% during this window is the real answer.

🌙 Step 3 — Nighttime Drift Test (6 PM–11 PM)

RR% should fall as outdoor temps drop.

If it stays high → insufficient capacity.

🌡 Step 4 — Humidity Behavior

Track humidity changes:

Oversized → humidity never drops
Undersized → humidity rises during long cycles
Correct → humidity stays between 45–55%

External Verified Source:

EPA humidity comfort guidelines

🌀 Step 5 — Cycle Duration Test

A correctly sized system cycles:

10–20 minutes per cycle
3–5 cycles per hour

Oversized systems → 3–8 minutes
Undersized systems → 30–60 minutes

📊 Step 6 — Delta-T Check (Supply vs Return)

A healthy cooling system should drop:

➡ 16–22°F

Low Delta-T → airflow issues
High Delta-T → oversizing or airflow restriction

This helps confirm your RR% interpretation.

7️⃣ 🧩 The Runtime Ratio Diagnostic Matrix (Samantha’s Interpretation Chart)

Use this chart to combine symptoms into a final diagnosis:

🟦 Diagnosis: OVERSIZED System

RR% < 30%
Feels cold then warm again
Humidity > 55%
Short cycles
Delta-T high (23–30°F)

Corrective Actions:

Reduce AC tonnage
Improve airflow
Add dehumidification
Use continuous fan mode
Increase static pressure relief

🟩 Diagnosis: PERFECTLY SIZED System

RR% 40–60%
Comfortable humidity
Even cooling across rooms
Stable operation

No changes needed.

🟥 Diagnosis: UNDERSIZED System

RR% 70–95%
Long cycles
Slow temperature recovery
Humidity rises during peak heat
Delta-T low (12–15°F)

Corrective Actions:

Increase tonnage by 0.5–1 ton
Improve duct airflow
Add return air
Reduce solar gain

8️⃣ 🧠 Why Runtime Ratio Is More Accurate Than Load Calculations

Manual J calculations are helpful — but they assume:

perfect insulation
perfect weather data
accurate shading info
perfect airflow
no duct leakage
consistent occupancy
estimated infiltration

Real homes don’t behave this cleanly.

Runtime Ratio is the actual truth of how your home performs.

It factors in:

your real duct losses
your true insulation
your actual window heat gain
real heat load
real airflow
real humidity
real family habits
real climate

It’s reality, not theory.

9️⃣ 📦 Using Runtime Ratio BEFORE Ordering a New System Online

This is where RR% becomes incredibly powerful:

You can match:

real-world load
real airflow
real humidity behavior

…to the correct equipment size before you buy it.

Here’s exactly how I use RR% for homeowners buying a new system:

IF RR% is BELOW 30% → DOWNSIZE 0.5–1 TON

A 4-ton becomes a 3.5-ton
A 3.5-ton becomes a 3-ton
A 3-ton becomes a 2.5-ton

R-32 systems (with higher efficiency) often justify an even larger reduction.

IF RR% is 40–60% → KEEP THE SAME SIZE

Perfect.
Replace like-for-like.

IF RR% is ABOVE 70% → UPSIZE by 0.5 TON

But…

Only IF airflow supports it.
If airflow is bad, fix ducts first, then retest RR%.

External Verified Source:

ACCA Manual S equipment selection standards

**🔟 ✔ Samantha’s Final Verdict:

Runtime Ratio Is the Only Trustworthy Sizing Method**

Oversizing is a comfort killer — and a money waster.
But with Runtime Ratio, you finally get a clear, data-driven answer to the most important HVAC question:

👉 Is my system too big, too small, or just right?

And because RR% reflects the actual load of your home, it eliminates the risk of guessing wrong when you order a new AC online.

If your RR% shows the system is oversized, downsizing will: